Same thing in OZ, it has been awhile since I worked on the tully, but i had to have my work visa dialed before getting there.
Specific to the tully, no license needed, but you have to train with them for a bit before you start to work. if you go to the tully there are two companies both of which are good, you will have the options of being based in Cairns and running the Tully as well as the Baron river, but traveling back and forth to the tully daily 1 1/2 hours each way. Go for being a tully guide based in Mission Beach. Smaller scene and awesome little town in the jungle on the water.
these rivers are dam controlled so they run year round, but their busy season is during the wet season which starts around oct/november and runs through march/april.
If you are a competent class IV guide that is comfortable on technical rivers you should be fine on the river stuff but the you need to train taking japanese customers down so you need to learn all of your basic river commands in japanese, then you need to learn all of the safety spots and the order in which you run and pull over to set up.